Self-Calibration of Source-Measure Unit via Capacitor

ABSTRACT

Systems and methods for calibration and operation of a source-measure unit (SMU). The system may include a functional unit and output terminals coupled to the functional unit. An excitation signal may be applied to a capacitor by the SMU. The capacitor may be included in a calibration circuit. The method may include obtaining one or more of a current calibration coefficient (CCC) or a voltage calibration coefficient (VCC). The CCC may correspond to a current-range setting and the VCC may correspond to a voltage-range setting. The CCC may be obtained from a value of a first current and a value of a second current developed in the capacitor responsive to the excitation signal. The VCC may be obtained from a value of a first voltage and a value of a second voltage developed across the capacitor responsive to the excitation signal.

FIELD OF THE INVENTION

The present invention relates generally to measurement and dataacquisition systems and, more particularly, to the calibration andoperation of source-measure units.

DESCRIPTION OF THE RELATED ART

Scientists and engineers often use measurement systems to perform avariety of functions, including measurement of a physical phenomenon, aunit under test (UUT) or device under test (DUT), test and analysis ofphysical phenomena, process monitoring and control, control ofmechanical or electrical machinery, data logging, laboratory research,and analytical chemistry, to name a few examples.

A typical measurement system comprises a computer system, which commonlyfeatures a measurement device, or measurement hardware. The measurementdevice may be a computer-based instrument, a data acquisition device orboard, a programmable logic device (PLD), an actuator, or other type ofdevice for acquiring or generating data. The measurement device may be acard or board plugged into one of the I/O slots of the computer system,or a card or board plugged into a chassis, or an external device. Forexample, in a common measurement system configuration, the measurementhardware is coupled to the computer system through a PCI bus, PXI (PCIextensions for Instrumentation) bus, a GPIB (General-Purpose InterfaceBus), a VXI (VME extensions for Instrumentation) bus, a serial port,parallel port, or Ethernet port of the computer system. Optionally, themeasurement system includes signal-conditioning devices, which receivefield signals and condition the signals to be acquired.

A measurement system may typically include transducers, sensors, orother detecting means for providing “field” electrical signalsrepresenting a process, physical phenomena, equipment being monitored ormeasured, etc. The field signals are provided to the measurementhardware. In addition, a measurement system may also typically includeactuators for generating output signals for stimulating a DUT.

Measurement systems, which may also be generally referred to as dataacquisition systems, may include the process of converting a physicalphenomenon (such as temperature or pressure) into an electrical signaland measuring the signal in order to extract information. PC-basedmeasurement and data acquisition (DAQ) systems and plug-in boards areused in a wide range of applications in the laboratory, in the field,and on the manufacturing plant floor, among others. Typically, in ameasurement or data acquisition process, analog signals are received bya digitizer, which may reside in a DAQ device or instrumentation device.The analog signals may be received from a sensor, converted to digitaldata (possibly after being conditioned) by an Analog-to-DigitalConverter (ADC), and transmitted to a computer system for storage and/oranalysis. Then, the computer system may generate digital signals thatare provided to one or more digital to analog converters (DACs) in theDAQ device. The DACs may convert the digital signal to an output analogsignal that is used, e.g., to stimulate a DUT.

Multifunction DAQ devices typically include digital I/O capabilities inaddition to the analog capabilities described above. Digital I/Oapplications may include monitoring and control applications, videotesting, chip verification, and pattern recognition, among others. DAQdevices may include one or more general-purpose, bidirectional digitalI/O lines to transmit and received digital signals to implement one ormore digital I/O applications. DAQ devices may also include aSource-Measure Unit (SMU), which may apply a voltage to a DUT andmeasure the resulting current, or may apply a current to the DUT andmeasure the resulting voltage. SMUs are typically configured to operateaccording to what is commonly referred to as “compliance limits”, tolimit the output current when sourcing voltage, and limit the outputvoltage when sourcing current. In other words, a compliance limit on themeasured signal may determine the (maximum) value of the sourced signal.For example, when applying a source voltage to a DUT and measuringcurrent, a given current value (e.g. 1 A) specified as the compliancelimit would determine the (maximum) input (source) voltage that might beprovided to the DUT. In most cases compliance limits may depend and/ormay be determined based on the DUTs, e.g. the maximum (absolute) valueof the current that may flow into the DUT, or the maximum (absolute)value of the voltage that may be applied across the terminals of theDUT.

In the case of most SMUs, the setpoint (the desired output voltage whensourcing and regulating voltage, or the desired current value whensourcing and regulating current) and the compliance limits are typicallyprogrammable. SMUs are available to cover a variety of signal levels,from the microvolt (μV) range to the kilovolt (kV) range, and from thefemtoampere (fA) range to the ampere (A) range. Some SMUs can deliver ordissipate significant power, while other SMUs may be operated at lowpower. The accuracy of SMUs is typically less than the accuracy ofhigh-quality calibrators and/or digital multi meters (DMMs).

Furthermore, some SMUs may rely on self-calibration to ensure thelong-term accuracy of the unit. Self-calibration may allow for theremoval of stability requirements for most of the circuitry of the SMU.Thus, instead of the majority of circuitry of the SMU requiring strictstability, with self-calibration, a small number of components may berequired to meet strict stability requirements. For example, instead ofrequiring an entire voltage measurement path to be stable over along-term, only a single voltage reference must be stable. Note,however, that a typical SMU may have multiple ranges of operation, oroperating ranges, and each may require its own reference or a method totransfer calibration from one operating range to another. In suchinstances, the self-calibration circuitry may become expensive andcomplex if the SMU includes many operating ranges or if the operatingranges span a large dynamic range.

For example, a prior art SMU may have voltage operating ranges, e.g.,voltage-range settings, of 1 volt (V) and 10V and current operatingranges, e.g., current-range settings, of 10 micro-ampere (μA), 100 μA, 1mili-ampere (mA), 10 mA, and 100 mA. Assume the 10V operating range maybe self-calibrated by measuring a known, stable 5V voltage reference.Thus, the 1V range may be calibrated by measuring an impedance, voltage(V) divided by current (I) (V/I), while using the 10V/1 mA operatingrange combination followed measuring the impedance while operating inthe 1V/1 mA operating range combination. Since the 10V operating rangehas been calibrated, the difference in the measurement of the impedancefor each operating range combination would be due to mis-calibration ofthe 1V operating range, which could then be corrected. Note, themeasured impedance may be a resistor with a known and stable impedance,e.g., a 1 kilo-ohm (kΩ) as known in the art. However, the 1 kΩ resistormay yield poor results when calibrating the 10 μA operating rangebecause only 10 mili-volts (mV), or 1% of the full-scale range of the 1Voperating range, would be required to achieve full-scale current on the10 μA operating range. Thus, for a proper self-calibration, multipleresistors may be required. Additionally, switching between the resistorsmay be complicated by the need to avoid introducing errors from theswitch resistance.

Other corresponding issues related to the prior art will become apparentto one skilled in the art after comparing such prior art with thepresent invention as described herein.

SUMMARY OF THE INVENTION

Various embodiments of a system and method for calibration and operationof a source-measure unit (SMU) are presented below. A system forcalibration and operation of the SMU may include a functional unit,output terminals, coupled to the functional unit, and a capacitor,selectively coupled to the functional unit in place of the outputterminals. In certain embodiments the capacitor may be included in acalibration circuit. The calibration circuit may include switchesconfigured to couple the capacitor to the functional unit in place ofthe output terminals. Additionally, in certain embodiments, the systemmay include additional functional units, e.g., one or more, or aplurality. Further, the system may be included in, or installed in, achassis. Accordingly, the functional unit, or one or more functionalunits, may be configured to perform the methods detailed below. Further,a non-transient computer memory medium may be configured to storeprogram instructions executable by the functional unit, one or morefunctional units, or a plurality of functional units to perform themethods detail below.

In an exemplary embodiment, a method for calibrating and operating theSMU may include applying an excitation signal by the SMU to a capacitorand obtaining one or more of a current calibration coefficient (CCC) ora voltage calibration coefficient (VCC). The CCC may correspond to acurrent-range setting of the SMU. The VCC may correspond to avoltage-range setting of the SMU.

Obtaining the CCC may include determining a value of a first currentthat may be developed in a capacitor responsive to the excitation signaland a value of a second current that may be developed in the capacitorresponsive to the excitation signal. The CCC may be determined from thevalue of the first current and the value of the second current.

Obtaining the VCC may include determining a value of a first voltagethat may be developed across the capacitor responsive to the excitationsignal and a value of a second voltage that may be developed across thecapacitor responsive to the excitation signal. The VCC may be determinedfrom the value of the first voltage and the value of the second voltage.

In some embodiments, obtaining the CCC may further include applying anexcitation signal that may have an amplitude to the capacitor. In suchembodiments, the first current and the second current may be developedin the capacitor responsive to the excitation signal. In suchembodiments, obtaining the VCC may further include applying theexcitation signal to the capacitor. The first voltage and the secondvoltage may be developed across the capacitor responsive to theexcitation signal.

Further, in certain embodiments, determining the value of the firstcurrent and determining the value of the first voltage may each includeoperating the SMU with a first current-range setting and a firstvoltage-range setting. In such embodiments, determining the value of thesecond current may include operating the SMU with a second current-rangesetting and the first voltage-range setting. Additionally, determiningthe value of the second voltage may include operating the SMU with thefirst current-range setting and a second voltage-range setting.

In an exemplary embodiment, the amplitude may be a reference voltage.Additionally, in some embodiments, the excitation signal may be a firstexcitation signal. Further, the first excitation signal may have a firstoperating frequency and the amplitude may be a first amplitude.

In other embodiments, the CCC may be a first CCC and the VCC may be afirst VCC. In such embodiments, the method may further include obtainingone or more of a second CCC or a second VCC. The second CCC maycorrespond to a third current-range setting of the SMU. The second VCCmay correspond to a third voltage-range setting of the SMU.

In such embodiments, obtaining the second CCC may include applying asecond excitation signal to the capacitor. The second excitation signalmay have a second operating frequency and a third current and a fourthcurrent may be developed in the capacitor responsive to the secondexcitation signal. Additionally, a value of the third current developedin the capacitor and a value of the fourth current developed in thecapacitor may be determined and the second CCC may be determined fromthe value of the third current and the value of the fourth current.

Additionally, in such embodiments, obtaining the second VCC may includeapplying the second excitation signal to the capacitor and a thirdvoltage and a fourth voltage may be developed across the capacitorresponsive to the second excitation signal. Further, a value of thethird voltage developed across the capacitor and a value of the fourthvoltage developed across the capacitor may be determined and the secondVCC may be determined from the value of the third voltage and the valueof the fourth voltage.

Accordingly, in certain embodiments, determining the value of the thirdcurrent and determining the value of the third voltage each may includeoperating the SMU with the first current-range setting and the firstvoltage-range setting. In such embodiments, determining the value of thefourth current may include operating the SMU with the thirdcurrent-range setting and the first voltage-range setting anddetermining the value of the fourth voltage may include operating theSMU with the first current-range setting and the third voltage-rangesetting.

In some embodiments, the second excitation signal may have a secondamplitude. In other embodiments, the excitation signal may be atrapezoidal waveform. In one embodiment, the operating frequency of theexcitation signal may be specified to maximize a signal-to-noise ratio.

In one embodiment, the determining the CCC may further includedetermining the CCC based on a first characteristic of the capacitorcorresponding to the value of the first current and a secondcharacteristic of the capacitor corresponding to the value of the secondcurrent. Accordingly, determining the VCC may further includedetermining the VCC based on a third characteristic of the capacitorcorresponding to the value of the first voltage and a fourthcharacteristic of the capacitor corresponding to the value of the secondvoltage. In such embodiments, the characteristic of the capacitor may beone of an impedance, a reactance, or a capacitance.

In an exemplary embodiment, the capacitor may have an impedance and themethod may further include determining the impedance of the capacitorfor a specified excitation frequency. Further, in certain embodiments,the capacitor may include a plurality of impedances based on operatingfrequency of the excitation signal and the method may further includedetermining one or more impedances corresponding to one or moreexcitation frequencies. In such embodiments, the method may also includeweighting each of the one or more impedances to offset the correspondingapparent series loss in the circuit at each of the one or moreexcitation frequencies.

In another embodiment, the excitation may have an amplitude and anoperating frequency and determining the CCC may further includedetermining the CCC based on a known characteristic of the capacitor andthe value of the first and second currents. Similarly, determining theVCC may further include determining the VCC based on the knowncharacteristic of the capacitor and the value of the first and secondvoltages. In such embodiments, the known characteristic of the capacitormay be based on the operating frequency of the excitation signal. Incertain embodiments, the known characteristic may include one of animpedance, a reactance, or a capacitance and the method may furtherinclude determining the known characteristic of the capacitor for aspecified excitation signal where the specified excitation signal has anoperating frequency.

In another exemplary embodiment, a method for calibrating an SMU mayinclude operating the SMU with a first current-range setting and a firstvoltage-range setting and operating the SMU with a second current-rangesetting and a second voltage-range setting. Operating the SMU with thefirst current-range setting and first voltage-range setting may includeapplying an excitation to a calibration circuit. In one embodiment, thecalibration circuit may include a capacitor. In one embodiment, a valueof a first impedance of the calibration circuit may be determinedresponsive to the excitation signal.

In certain embodiments, the excitation signal may include an amplitude.Additionally, the excitation signal may include an operating frequency.The excitation signal may be one of a plurality of excitation signals.In such embodiments, each of the plurality, or one or more, excitationsignals may include a corresponding or associated amplitude and acorresponding or associated operating frequency.

Operating the SMU with the second current-range setting and the secondvoltage-range setting may include applying the excitation signal to thecalibration circuit. In one embodiment, a value of a second impedance ofthe calibration circuit may be determined responsive to the excitationsignal.

Additionally, a calibration coefficient corresponding to the secondcurrent-range setting and second voltage-range setting of the SMU may bedetermined. In one embodiment, the calibration coefficient may bedetermined from the value of the first impedance and the value of thesecond impedance.

In yet another exemplary embodiment, a method for calibrating andoperating a source-measure unit (SMU), may include applying anexcitation signal by the SMU to a capacitor. The excitation signal mayhave an amplitude and operating frequency. In certain embodiments, theexcitation signal may have a trapezoidal waveform. In such embodiments,the operating frequency of the excitation signal may be the fundamentalfrequency of the trapezoidal waveform. In certain embodiments, theamplitude and operating frequency of the excitation signal may bespecified to maximize a signal-to-noise ratio.

Responsive to the excitation signal, a current may developed in thecapacitor and voltage may be developed across the capacitor.Accordingly, a value of the current and a value of the voltage may bedetermined. Further, one or more of a current calibration coefficient(CCC) or a voltage calibration coefficient (VCC) may be obtained. TheCCC may correspond to a current-range setting of the SMU and may bedetermined from the value of the current, the value of the voltage, anda known characteristic of the capacitor. Similarly, the VCC maycorrespond to a voltage-range setting of the SMU and may be determinedfrom the value of the voltage, the value of the current, and the knowncharacteristic of the capacitor.

In one embodiment of the method, the known characteristic of thecapacitor may be one of an impedance, a reactance, or a capacitance. Incertain embodiments, the capacitor may have one or more knowncharacteristics that may correspond to one or more frequencies of theexcitation signal. In such embodiments, the method may further includeweighting each of the one or more known characteristics to offset acorresponding one or more apparent series losses in the circuit. Note,the one or more apparent series losses may be a function of the one ormore frequencies of the excitation signal.

Further, in some embodiments, the method may also include determiningthe known characteristic of the capacitor for a specified excitationsignal. Note, in such embodiments, the specified excitation signal mayhave an operating frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 illustrates an instrumentation system according to one embodimentof the invention;

FIG. 2 is a block diagram of an exemplary system according to oneembodiment of the invention;

FIG. 3 is a flowchart diagram illustrating one embodiment of a methodfor self-calibrating an SMU; and

FIG. 4 is a flowchart diagram illustrating another embodiment of amethod for self-calibrating an SMU;

FIG. 5 is a flowchart diagram illustrating another embodiment of amethod for self-calibrating an SMU; and

FIGS. 6A and 6B illustrate exemplary voltage and current signalsresponsive to an excitation signal applied to a capacitor by an SMU.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and are herein described in detail. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION Incorporation by Reference

The following reference is hereby incorporated by reference in itsentirety as though fully and completely set forth herein:

U.S. Pat. No. 7,903,008 titled “Source-Measure Unit Based on DigitalControl Loop,” issued on Nov. 6, 2008.

TERMS

The following is a glossary of terms used in the present application:

Memory Medium—Any of various types of non-transitory computer accessiblememory devices or storage devices. The term “memory medium” is intendedto include an installation medium, e.g., a CD-ROM, floppy disks 104, ortape device; a computer system memory or random access memory such asDRAM, DDR RAM, SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memorysuch as a Flash, magnetic media, e.g., a hard drive, or optical storage;registers, or other similar types of memory elements, etc. The memorymedium may comprise other types of non-transitory memory as well orcombinations thereof. In addition, the memory medium may be located in afirst computer in which the programs are executed, or may be located ina second different computer which connects to the first computer over anetwork, such as the Internet. In the latter instance, the secondcomputer may provide program instructions to the first computer forexecution. The term “memory medium” may include two or more memorymediums which may reside in different locations, e.g., in differentcomputers that are connected over a network.

Carrier Medium—a memory medium as described above, as well as a physicaltransmission medium, such as a bus, network, and/or other physicaltransmission medium that conveys signals such as electrical,electromagnetic, or digital signals.

Programmable Hardware Element—includes various hardware devicescomprising multiple programmable function blocks connected via aprogrammable interconnect. Examples include FPGAs (Field ProgrammableGate Arrays), PLDs (Programmable Logic Devices), FPOAs (FieldProgrammable Object Arrays), and CPLDs (Complex PLDs). The programmablefunction blocks may range from fine grained (combinatorial logic or lookup tables) to coarse grained (arithmetic logic units or processorcores). A programmable hardware element may also be referred to as“reconfigurable logic”.

Software Program—the term “software program” is intended to have thefull breadth of its ordinary meaning, and includes any type of programinstructions, code, script and/or data, or combinations thereof, thatmay be stored in a memory medium and executed by a processor. Exemplarysoftware programs include programs written in text-based programminglanguages, such as C, C++, PASCAL, FORTRAN, COBOL, JAVA, assemblylanguage, etc.; graphical programs (programs written in graphicalprogramming languages); assembly language programs; programs that havebeen compiled to machine language; scripts; and other types ofexecutable software. A software program may comprise two or moresoftware programs that interoperate in some manner. Note that variousembodiments described herein may be implemented by a computer orsoftware program. A software program may be stored as programinstructions on a memory medium.

Hardware Configuration Program—a program, e.g., a netlist or bit file,that can be used to program or configure a programmable hardwareelement.

Program—the term “program” is intended to have the full breadth of itsordinary meaning. The term “program” includes 1) a software programwhich may be stored in a memory and is executable by a processor or 2) ahardware configuration program useable for configuring a programmablehardware element.

Graphical Program—A program comprising a plurality of interconnectednodes or icons, wherein the plurality of interconnected nodes or iconsvisually indicate functionality of the program. The interconnected nodesor icons are graphical source code for the program. Graphical functionnodes may also be referred to as blocks.

The following provides examples of various aspects of graphicalprograms. The following examples and discussion are not intended tolimit the above definition of graphical program, but rather provideexamples of what the term “graphical program” encompasses:

The nodes in a graphical program may be connected in one or more of adata flow, control flow, and/or execution flow format. The nodes mayalso be connected in a “signal flow” format, which is a subset of dataflow.

Exemplary graphical program development environments which may be usedto create graphical programs include LabVIEW®, DasyLab™, DIADem™ andMatrixx/SystemBuild™ from National Instruments, Simulink® from theMathWorks, VEE™ from Agilent, WiT™ from Coreco, Vision Program Manager™from PPT Vision, SoftWIRE™ from Measurement Computing, Sanscript™ fromNorthwoods Software, Khoros™ from Khoral Research, SnapMaster™ from HEMData, VisSim™ from Visual Solutions, ObjectBench™ by SES (Scientific andEngineering Software), and VisiDAQ™ from Advantech, among others.

The term “graphical program” includes models or block diagrams createdin graphical modeling environments, wherein the model or block diagramcomprises interconnected blocks (i.e., nodes) or icons that visuallyindicate operation of the model or block diagram; exemplary graphicalmodeling environments include Simulink®, SystemBuild™, VisSim™,Hypersignal Block Diagram™, etc.

A graphical program may be represented in the memory of the computersystem as data structures and/or program instructions. The graphicalprogram, e.g., these data structures and/or program instructions, may becompiled or interpreted to produce machine language that accomplishesthe desired method or process as shown in the graphical program.

Input data to a graphical program may be received from any of varioussources, such as from a device, unit under test, a process beingmeasured or controlled, another computer program, a database, or from afile. Also, a user may input data to a graphical program or virtualinstrument using a graphical user interface, e.g., a front panel.

A graphical program may optionally have a GUI associated with thegraphical program. In this case, the plurality of interconnected blocksor nodes are often referred to as the block diagram portion of thegraphical program.

Computer System—any of various types of computing or processing systems,including a personal computer system (PC), mainframe computer system,workstation, network appliance, Internet appliance, personal digitalassistant (PDA), television system, grid computing system, or otherdevice or combinations of devices. In general, the term “computersystem” can be broadly defined to encompass any device (or combinationof devices) having at least one processor that executes instructionsfrom a memory medium.

Measurement Device—includes instruments, data acquisition devices, smartsensors, and any of various types of devices that are configured toacquire and/or store data. A measurement device may also optionally befurther configured to analyze or process the acquired or stored data.Examples of a measurement device include an instrument, such as atraditional stand-alone “box” instrument, a computer-based instrument(instrument on a card) or external instrument, a data acquisition card,a device external to a computer that operates similarly to a dataacquisition card, a smart sensor, one or more DAQ or measurement cardsor modules in a chassis, an image acquisition device, such as an imageacquisition (or machine vision) card (also called a video capture board)or smart camera, a motion control device, a robot having machine vision,and other similar types of devices. Exemplary “stand-alone” instrumentsinclude oscilloscopes, multimeters, signal analyzers, arbitrary waveformgenerators, spectroscopes, and similar measurement, test, or automationinstruments.

A measurement device may be further configured to perform controlfunctions, e.g., in response to analysis of the acquired or stored data.For example, the measurement device may send a control signal to anexternal system, such as a motion control system or to a sensor, inresponse to particular data. A measurement device may also be configuredto perform automation functions, i.e., may receive and analyze data, andissue automation control signals in response.

Functional Unit (or Processing Element)—refers to various elements orcombinations of elements. Processing elements include, for example,circuits such as an ASIC (Application Specific Integrated Circuit),portions or circuits of individual processor cores, entire processorcores, individual processors, programmable hardware devices such as afield programmable gate array (FPGA), and/or larger portions of systemsthat include multiple processors, as well as any combinations thereof.

Automatically—refers to an action or operation performed by a computersystem (e.g., software executed by the computer system) or device (e.g.,circuitry, programmable hardware elements, ASICs, etc.), without userinput directly specifying or performing the action or operation. Thusthe term “automatically” is in contrast to an operation being manuallyperformed or specified by the user, where the user provides input todirectly perform the operation. An automatic procedure may be initiatedby input provided by the user, but the subsequent actions that areperformed “automatically” are not specified by the user, i.e., are notperformed “manually”, where the user specifies each action to perform.For example, a user filling out an electronic form by selecting eachfield and providing input specifying information (e.g., by typinginformation, selecting check boxes, radio selections, etc.) is fillingout the form manually, even though the computer system must update theform in response to the user actions. The form may be automaticallyfilled out by the computer system where the computer system (e.g.,software executing on the computer system) analyzes the fields of theform and fills in the form without any user input specifying the answersto the fields. As indicated above, the user may invoke the automaticfilling of the form, but is not involved in the actual filling of theform (e.g., the user is not manually specifying answers to fields butrather they are being automatically completed). The presentspecification provides various examples of operations beingautomatically performed in response to actions the user has taken.

Concurrent—refers to parallel execution or performance, where tasks,processes, or programs are performed in an at least partiallyoverlapping manner. For example, concurrency may be implemented using“strong” or strict parallelism, where tasks are performed (at leastpartially) in parallel on respective computational elements, or using“weak parallelism”, where the tasks are performed in an interleavedmanner, e.g., by time multiplexing of execution threads.

Operating Range—refers to the range of operation of the source-measureunit (SMU). Typically referred to as a single value which denotes thefull-scale value of the range, e.g., 10V range refers to the operatingvoltage-range of 0-10V and 10 mA range refers to the operatingcurrent-range of 0-10 mA. Note, the operating range of the SMU may beexpressed in terms of either a current-range (for example, 10 fA), avoltage-range (for example, 60 V), or both (60 V/10 fA).

Current-Range Setting—refers to the operating range of the SMU in termsof current. For example, an SMU may have current-range settings of 1 A,100 mA, 10 mA, 1 mA, 100 μA, 10 μA, and 1 μA, among others. Further,current-range setting may by uni-polar or bi-polar. In other words, thecurrent-range settings may designate a range of ±1 A or 0-1 A, ±100 mAor 0-100 mA, ±10 mA or 0-10 mA, ±1 mA or 0-1 mA, and so forth.

Voltage-Range Setting—refers to the operating range of thesource-measure unit in terms of voltage. For example, an SMU may havevoltage-range settings of 60V, 10V, 6V, 1V, and 200 mV, among others.Further, voltage-range settings may be uni-polar or bi-polar. In otherwords, the voltage-range settings may designate a range of ±60V or0-60V, ±10V or 0-10V, ±6V or 0-6V, and so forth.

Calibration Coefficient—refers to a gain correction value associatedwith correcting the error in measurement, either voltage or current, ofa signal. If the gain correction value is associated with a current, itmay be referred to as a current calibration coefficient (CCC). If thegain correction value is associated with a voltage, it may be referredto as a voltage calibration coefficient (VCC). In general, eachoperating range of an SMU may have an associated calibrationcoefficient.

Excitation Signal—refers to an alternating current (A/C) signal that maybe defined by an amplitude (voltage) and frequency.

FIG. 1—Exemplary Instrumentation System

FIG. 1 illustrates an exemplary instrumentation system 100 configuredwith embodiments of the present invention. Embodiments of the presentinvention may be involved with performing test and/or measurementfunctions; controlling and/or modeling instrumentation or industrialautomation hardware; modeling and simulation functions, e.g., modelingor simulating a device or product being developed or tested, etc.However, it is noted that embodiments of the present invention can beused for a plethora of applications and is not limited to the aboveapplications. In other words, applications discussed in the presentdescription are exemplary only, and embodiments of the present inventionmay be used in any of various types of systems.

As shown in FIG. 1, the system 100 may include a host computer 82. Thehost computer 82 may be coupled to a network and include a displaydevice and at least one memory medium on which one or more computerprograms or software components, according to one embodiment of thepresent invention may be stored. For example, the memory medium maystore one or more graphical programs which are executable to perform themethods described herein. Additionally, the memory medium may store agraphical programming development environment application used to createand/or execute such graphical programs. The memory medium may also storeoperating system software, as well as other software for operation ofthe computer system. Various embodiments further include receiving orstoring instructions and/or data implemented in accordance with theforegoing description upon a carrier medium.

Further, the host computer 82 may include a central processing unit(CPU) and one or more input devices such as a mouse or keyboard asshown. The computer 82 may operate with the one or more instruments toanalyze, measure or control a unit under test (UUT) 150, e.g., viaexecution of software 104.

The one or more instruments may include PXI instrument 118. PXIinstrument 118 may include a source-measure unit (SMU) according toembodiments of the present invention. Alternatively, the SMU may beincluded in another type of chassis or may by a stand-alone, orindependent, device. The computer system may couple to and operate withPXI instrument 118. PXI instrument 118 may be coupled to the UUT 150.The system 100 may be used in a data acquisition and control applicationor in a test and measurement application, among others. Additionally,PXI instrument 118 may couple to host computer 82 over a network, suchas the Internet.

FIG. 2—SMU Block Diagram

FIG. 2 is a block diagram of an exemplary system according to oneembodiment of the invention. FIG. 2 illustrates the basic architectureof one embodiment of an SMU 200 in which the entire control loop hasbeen configured in the digital domain. A device under test (DUT), notshown, may be coupled between output terminals 220 and 222. Setpointsand compliance limits may be provided (programmed) to Digital ControlLoop (DCL) 202, which may provide a control output through DAC(digital-to-analog converter) 204 to Output Stage 210. Feedback fromOutput Stage 210 may be provided to Current ADC (analog-to-digitalconverter) 206 and Voltage ADC 208 via respective Current Sense element212 and Voltage Sense element 214. As shown, different current shuntresistors 216 may be switched into the feedback loop between the outputof Output Stage 210 and the inputs of Current Sense element 212, using amultiplexer 236 and a set of switches 234. Although FIG. 2 shows threeswitches (234) and three current shunt resistors (216), alternateembodiments may be configured with a greater or lesser number ofswitches and/or resistors, as desired. Shunt switching may provide theSMU with the capability to cover a wider dynamic range of current. Anyglitches that may result from switching between the various currentshunt resistors may be minimized by adjusting the settings of DAC 204simultaneously with the shunt-switching operation. Since the current isbeing measured and the values of the current shunt resistors (216) areknown, it is possible to calculate the value to which DAC 204 may be setto minimize potential glitches. Any errors in the calculations mayeventually be corrected by DCL 202. Thus, Current ADC 206 and VoltageADC 208 may then provide the readback current and voltage values intoDCL 202. Additionally, SMU 200 may include a capacitor 218 which may beselectively switched into the circuit in place of the output terminals220 and 222 during a self-calibration of the SMU 200.

Further, DCL 202 may include a functional unit. As used herein, the termfunctional unit refers to various elements or combinations of elements.Processing elements include, for example, circuits such as an ASIC(Application Specific Integrated Circuit), portions or circuits ofindividual processor cores, entire processor cores, individualprocessors, programmable hardware devices such as a field programmablegate array (FPGA), and/or larger portions of systems that includemultiple processors, as well as any combinations thereof. In oneembodiment, the functional unit may be configured to perform the methodsdescribed below. In certain embodiments, a memory medium may store themethods described below for execution on one or more processing units,such as the functional unit that may be included in DCL 202.

Thus, in certain embodiments, the functional unit may be configured toobtain one or more of a current calibration coefficient (CCC)corresponding to a first current-range of SMU 200 or a voltagecalibration coefficient (VCC) corresponding to a first voltage-range ofSMU 200. In order to obtain the CCC, the functional unit may beconfigured to determine a value of a first current developed incapacitor 218 and a value of a second current developed in capacitor218. In certain embodiments, the first and second current may bedeveloped in capacitor 218 responsive to an excitation signal that maybe applied by SMU 200 to capacitor 218. Further, the functional unit maybe configured to determine the CCC from the value of the first currentand the value of the second current.

Accordingly, in order to obtain the VCC, the functional unit may beconfigured to determine a value of a first voltage developed across thecapacitor 218 and a value of a second voltage developed across thecapacitor 218. In certain embodiments, the first and second voltage maybe developed across capacitor 218 responsive to an excitation signalthat may be applied by SMU 200 to capacitor 218. Additionally, thefunctional unit may be configured to determine the VCC from the value ofthe first voltage and the value of the second voltage.

As noted above, in one embodiment, obtaining the CCC may also includeapplying an excitation signal to the capacitor 218. Accordingly, thefirst current and the second current may be developed in the capacitorin response to the excitation signal. Note, the term excitation signalis used to refer to an alternating current (A/C) signal that may bedefined by an amplitude (voltage) and operating frequency. Thus, in someembodiments, the excitation signal may have an amplitude. The amplitudeof the excitation signal may be expressed in terms of a voltage level.Thus, for example, an excitation signal may have a 1 V amplitude or a 5V amplitude, among others. In certain embodiments, the amplitude may beprecisely known. In other words, the source of the excitation signal mayinclude a known and stable voltage source. Further, the excitationsignal may also have an operating frequency. The operating frequency maybe expressed in hertz (Hz). Thus, the excitation signal may be specifiedby both an amplitude and an operating frequency. In certain embodiments,the excitation signal may be a trapezoidal waveform. In suchembodiments, the excitation signal may be expressed as a series ofsinusoidal waveforms, each defined by a frequency. Thus the excitationsignal may have more than one frequency. In such embodiments, theoperating frequency of the excitation signal may be expressed in termsof the fundamental frequency of the trapezoidal waveform. In otherembodiments, the excitation frequency may be specified in order tomaximize the signal-to-noise ratio (SNR) of the signal. In an exemplaryembodiment, the excitation signal may be one of a plurality ofexcitation signals. Thus, each excitation signal of the plurality ofexcitation signals may be specified by an amplitude and an operatingfrequency.

For example, a first excitation signal may include a first amplitude anda first operating frequency. Accordingly, a second excitation signal mayinclude a second amplitude and a second operating frequency. Note, insome embodiments, one or more excitation signals may include equivalentamplitudes but have different operating frequencies. Similarly, one ormore excitation signals may include equivalent operating frequencies buthave different amplitudes. Thus, in some embodiments, the first andsecond amplitudes may be equivalent, whereas, in other embodiments, thefirst and second operating frequencies may be equivalent.

In another embodiment, obtaining the VCC may also include applying anexcitation signal to the capacitor 218. Accordingly, the first voltageand the second voltage may be developed across the capacitor in responseto the excitation signal.

In an exemplary embodiment, self-calibration may include operating theSMU 200 at multiple, e.g., one or more, or a plurality of, current-rangesettings and voltage-range settings. In such embodiments, in order todetermine a CCC for a particular current-range setting, a firstcurrent-range may have been calibrated using a known prior art method,e.g., for example, via a reference resistor with a known and stableimpedance. Similarly, in order to determine a VCC for a particularvoltage-range setting, a first voltage-range setting may have beencalibrated using a known prior art method, e.g., for example, via areference voltage from a known and stable source.

Thus, the SMU 200 may be operated in a first current-range setting and afirst voltage-range setting in order to determine the value of the firstcurrent and the value of the first voltage. Accordingly, the SMU 200 maybe operated in a second current-range setting and the firstvoltage-range setting in order to determine the value of the secondcurrent. Similarly, the SMU 200 may be operating in the firstcurrent-range setting and a second voltage-range setting in order todetermine the value of the second voltage.

For example, a 6V range of an SMU, such as SMU 200, may be calibratedusing a known 5V voltage reference. In other words, a firstvoltage-range setting may be calibrated using a reference voltage.Further, a 100 μA to range of an SMU, such as SMU 200, may be calibratedusing the voltage reference and a known reference resistor, e.g., aresistor with a known and stable impedance. In other words, a firstcurrent-range setting may be calibrated using a reference voltage and areference resistor. Accordingly, a 600 mV range may then be calibratedbased on the 6V range and 100 μA to range. In other words, a secondvoltage-range setting may be calibrated based on the first voltage-rangesetting and first current-range setting. Similarly, a 1 mA range maythen be calibrated based on the 6V range and 100 μA to range. In otherwords, a second current-range setting may be calibrated based on thefirst voltage-range setting and first current-range setting.

Continuing the example, the value of the first current and the value ofthe first voltage may be determined by operating the SMU 200 with the 6V/100 μA to range setting and applying an excitation signal to thecapacitor 218. The value of the second voltage may be determined byoperating the SMU 200 with the 600 mv/100 μA to range setting andapplying the excitation signal to the capacitor 218. The value of thesecond current may be determined by operating the SMU 200 with the 6 V/1mA range setting and applying the excitation signal to the capacitor218. Further, the excitation signal may be a first excitation signal andmay include a first operating frequency and first amplitude. Note, inorder to calibrate, e.g., obtain calibration coefficients for, othervoltage-range settings and current-range settings may require otherexcitation signals that may include other amplitudes and operatingfrequencies. Thus, a plurality of CCCs and VCCs may be determined.

In certain embodiments, the capacitor 218 may have an impedance, such as1 kilo-ohm (1 kΩ). However, the impedance of the capacitor 218 may bedependent upon the operating frequency of the excitation signal. Thus,the impedance of the capacitor for a specified excitation frequency maybe determined. In other embodiments, one or more impedancescorresponding to one or more operating, or excitation, frequencies maybe determined. Thus the capacitor 218 may have a plurality of impedancesbased on excitation frequency. Further, in some embodiments, each of theone or more impedances of the capacitor 218 may be weighted to offsetthe corresponding apparent series loss in the circuit at each of the oneor more excitation frequencies

In certain embodiments, capacitor 218 may have a capacitance that issubstantially constant with frequency. In other words, the capacitancemay be stable, or assumed to not change, with frequency. In suchembodiments, other characteristics of the capacitor may changeproportionally with frequency. For example, the impedance, and morespecifically the reactance of capacitor 218 may decrease in proportionto the operating frequency of the excitation signal. Thus, in certainembodiments, the calibration coefficients, e.g., the CCC and VCC, may bedetermined from the one of the reactance or capacitance of capacitor218.

For example, in an embodiment where excitation signal may apply avoltage to the capacitor, only current in quadrature to the appliedvoltage at a given frequency may be used to determine the CCC. Thus, anycurrent in phase with the applied voltage may be ignored because it maybe due to parasitic resistance and, therefore, not dependable. Thus, asa further example, assume an excitation signal with a trapezoidalwaveform may be applied to capacitor 218. Then, the voltage acrosscapacitor 218 and the current in capacitor 218 may be determined byvoltage ADC 208 and current ADC 206, respectively. Accordingly, a FastFourier Transform (FFT) may be performed to extract a magnitude andphase of the voltage and current at the fundamental frequency, e.g.,operating frequency, of the trapezoidal waveform. Further, in certainembodiments, the FFT may be performed to extract magnitude and phase ofthe voltage and current at one or more harmonics of the trapezoidalwaveform. Thus, dividing the determined voltage by the current maydetermine the total measured impedance of the capacitor at the specifiedfrequency. Accordingly, the imaginary part of this complex number may bethe measured reactance of the capacitor. Thus, the apparent change inthis reactance when switching ranges may be used to determine the CCC orVCC. Additionally, by using the reactance (or the linearly-relatedcapacitance) sensitivity to resistances in the circuit, such as thevalues of the current-range-setting shunt resistors 216 may be removed,since resistances impact only the real part of the measured impedance.

In another exemplary embodiment, an excitation signal may be applied tocapacitor 218 by SMU 200. In such embodiments, a value of a currentdeveloped in capacitor 218 responsive to the excitation signal may bedetermined. Further, a value of a voltage developed across capacitor 218responsive to the excitation signal may be determined. Then, one or moreof a CCC or a VCC may be obtained. The CCC may correspond to acurrent-range setting of SMU 200 and may be determined from the value ofthe current, the value of the voltage, and a known characteristic ofcapacitor 218. Similarly, the VCC may correspond to a voltage-rangesetting of SMU 200 and may be determined from the value of the voltage,the value of the current, and the known characteristic of capacitor 218.

In one embodiment, the known characteristic of capacitor 218 may be oneof a capacitance, an impedance, or a reactance. The term knowncharacteristic refers to a quantifiable characteristic that may beconsidered stable for the useful life of the capacitor. Thus, in oneembodiment, the capacitor may be characterized over one or moreoperating frequencies. In other words, the known characteristic maychange in proportion to operating frequency of an excitation signal andthe known characteristic may be determined for one or more specifiedoperating frequencies.

FIG. 3—Flowchart of a Method for Calibrating and Operating an SMU

FIG. 3 illustrates a method for calibrating and operating asource-measure unit (SMU). The method shown in FIG. 3 may be used inconjunction with any of the computer systems or devices shown in theabove Figures, among other devices. Further, a non-transient computermemory medium may be configured to store program instructions executableby one or more functional units to perform the method. In variousembodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. As shown,this method may operate as follows.

In 302, an excitation signal may be applied to a capacitor by an SMU. Asmentioned, the SMU may be the similar or the same as SMU 200 describedabove. Similarly, the capacitor may be similar or the same as capacitor218 described above. In some embodiments, the excitation signal mayinclude a trapezoidal waveform. In certain embodiments, the excitationsignal may be a trapezoidal waveform. In such embodiments, theexcitation signal may be expressed as a series of sinusoidal waveforms,each defined by a frequency. Thus the excitation signal may have morethan one frequency. In such embodiments, the operating frequency of theexcitation signal may be expressed in terms of the fundamental frequencyof the trapezoidal waveform. Additionally, in certain embodiments, theoperating frequency of the excitation signal may be specified tomaximize a signal-to-noise ratio.

Then, one or more (at least one) of a current calibration coefficient(CCC) or a voltage calibration coefficient (VCC) may be obtained. TheCCC may have a corresponding current-range setting. The VCC may have acorresponding voltage-range setting. In other words, the CCC maycorrespond to a first current-range setting and the VCC may correspondto a first voltage-range setting.

In 304, a CCC may be obtained as illustrated in 314 and 324.

In 314, a value of a first current developed in a capacitor responsiveto the excitation signal may be determined. Additionally, a value of asecond current developed in the capacitor responsive to the excitationsignal may be determined. Note, that the SMU may be similar to SMU 200and may be included in a system similar to System 100. Additionally, thecapacitor may be similar to capacitor 218. Further, in certainembodiments, an excitation signal which may have an amplitude may beapplied to the capacitor. Note, in certain embodiments, the amplitudemay include a reference voltage. In such embodiments, the first currentand the second current may be developed in the capacitor responsive tothe excitation signal. Additionally, in some embodiments, the SMU may beoperated in a first current-range setting and a first voltage-rangesetting to determine the value of the first current. In suchembodiments, the SMU may be operated in a second current-range settingand the first voltage-range setting to determine the value of the secondcurrent.

In 324, the CCC may be determined from the value of the first currentand the value of the second current. In certain embodiments, the CCC maybe a first CCC. In other words, in certain embodiments there may be oneor more, or a plurality of, CCCs. In such embodiments, the method mayfurther include obtaining a second CCC. The second CCC may correspond toa third current-range setting of the SMU. Obtaining the second CCC mayinclude applying a second excitation signal to the capacitor. The secondexcitation signal may include a second operating frequency. Further, thesecond excitation signal may have a second amplitude. Additionally, athird current and a fourth current may be developed in the capacitorresponsive to the second excitation signal. Further, a value of thethird current developed in the capacitor and a value of the fourthcurrent developed in the capacitor may be determined and the second CCCmay be determined from the value of the third current and the value ofthe fourth current. In certain embodiments, the value of the thirdcurrent may be determined by operating the SMU with the firstcurrent-range setting and the first voltage-range setting and the valueof the fourth current may be determined by operating the SMU with thethird current-range setting and the first voltage-range setting.

In 306, a VCC may be obtained as illustrated in 316 and 326.

In 316, a value of a first voltage developed across the capacitorresponsive to the excitation signal may be determined. Additionally, avalue of a second voltage developed across the capacitor responsive tothe excitation signal may be determined. Note, that the SMU may besimilar to SMU 200 and may be included in a system similar to System100. Additionally, the capacitor may be similar to capacitor 218.Further, in certain embodiments, an excitation signal which may includean amplitude may be applied to the capacitor. Note, in certainembodiments, the amplitude may include a reference voltage. In suchembodiments, the first voltage and the second voltage may be developedacross the capacitor responsive to the excitation signal. Additionally,in some embodiments, the SMU may be operated in the first current-rangesetting and the first voltage-range setting to determine the value ofthe first voltage. In such embodiments, the SMU may be operated in thefirst current-range setting and a second voltage-range setting todetermine the value of the second voltage.

In 326, the VCC may be determined from the value of the first voltageand the value of the second voltage. In certain embodiments, the VCC maybe a first VCC. In other words, in certain embodiments there may be oneor more, or a plurality of, VCCs. In such embodiments, the method mayfurther include obtaining a second VCC. The second VCC may correspond toa third voltage-range setting of the SMU. Obtaining the second VCC mayinclude applying a second excitation signal to the capacitor. The secondexcitation signal may include a second operating frequency. Further, thesecond excitation signal may include a second amplitude. Additionally, athird voltage and a fourth voltage may be developed across the capacitorresponsive to the second excitation signal. Further, a value of thethird voltage developed in the capacitor and a value of the fourthvoltage developed in the capacitor may be determined and the second VCCmay be determined from the value of the third voltage and the value ofthe fourth voltage. In certain embodiments, the value of the thirdvoltage may be determined by operating the SMU with the firstcurrent-range setting and the first voltage-range setting and the valueof the fourth voltage may be determined by operating the SMU with thefirst current-range setting and the third voltage-range setting.

Further, the capacitor may include an impedance and the method mayfurther include determining the impedance of the capacitor for aspecified excitation frequency. Additionally, in an exemplaryembodiment, the capacitor may include a plurality of impedances based onoperating frequency of the excitation signal. In such embodiments, themethod may further include determining one or more impedancescorresponding to one or more excitation frequencies. The method may alsoinclude weighting each of the one or more impedances to offset thecorresponding apparent series loss in the circuit at each of the one ormore excitation frequencies.

In certain embodiments, the capacitor may have a capacitance that issubstantially constant with frequency. In other words, the capacitancemay be stable, or assumed to not change, with frequency. In suchembodiments, other characteristics of the capacitor may changeproportionally with frequency. For example, the impedance, and morespecifically the reactance of the capacitor may decrease in proportionto the operating frequency of the excitation signal. Thus, in certainembodiments, the calibration coefficients, e.g., the CCC and VCC, may bedetermined from the one of the reactance or capacitance of thecapacitor.

FIG. 4—Flowchart of a Method for Calibrating and Operating an SMU

FIG. 4 illustrates another method for calibrating and operating asource-measure unit (SMU). The method shown in FIG. 4 may be used inconjunction with any of the computer systems or devices shown in theabove Figures, among other devices. Further, a non-transient computermemory medium may be configured to store program instructions executableby one or more functional units to perform the method. In variousembodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. As shown,this method may operate as follows.

In 402, the SMU, which may be similar to SMU 200, may be operated with afirst current-range setting and first voltage-range setting.

In 412, an excitation signal which may include an amplitude may beapplied to a calibration circuit including a capacitor. The capacitormay be similar to capacitor 218.

In 422, a value of a first impedance of the calibration circuit may bedetermined. The value of the first impedance may be responsive to theexcitation signal.

In 404, the SMU may be operated with a second current-range setting anda second voltage-range setting.

In 414, the excitation signal which may include the amplitude may beapplied to the calibration circuit including the capacitor.

In 424, a value of a second impedance of the calibration circuit may bedetermined. The value of the second impedance may be responsive to theexcitation signal.

In 406, a calibration coefficient (CC) may be obtained. The CC maycorrespond to the second current-range setting and the secondvoltage-range setting of the SMU. The CC may be determined from thevalue of the first impedance and the value of the second impedance.

FIG. 5—Flowchart of a Method for Calibrating and Operating an SMU

FIG. 5 illustrates another method for calibrating and operating asource-measure unit (SMU). The method shown in FIG. 5 may be used inconjunction with any of the computer systems or devices shown in theabove Figures, among other devices. Further, a non-transient computermemory medium may be configured to store program instructions executableby one or more functional units to perform the method. In variousembodiments, some of the method elements shown may be performedconcurrently, in a different order than shown, or may be omitted.Additional method elements may also be performed as desired. As shown,this method may operate as follows.

In 502, an excitation signal may be applied to a capacitor by an SMU. Asmentioned, the SMU may be the similar or the same as SMU 200 describedabove. Similarly, the capacitor may be similar or the same as capacitor218 described above. In some embodiments, the excitation signal mayinclude a trapezoidal waveform. In certain embodiments, the excitationsignal may be a trapezoidal waveform. In such embodiments, theexcitation signal may be expressed as a series of sinusoidal waveforms,each defined by a frequency. Thus the excitation signal may have morethan one frequency. In such embodiments, the operating frequency of theexcitation signal may be expressed in terms of the fundamental frequencyof the trapezoidal waveform. Additionally, in certain embodiments, theoperating frequency of the excitation signal may be specified tomaximize a signal-to-noise ratio.

In 504, a value of a current developed in the capacitor responsive tothe excitation signal may be determined.

In 506, a value of a voltage developed across the capacitor responsiveto the excitation signal may be determined.

In 508 or 510, one or more of a current calibration coefficient (CCC)corresponding to a current-range setting of the SMU or a voltagecalibration coefficient (VCC) corresponding to a voltage-range settingof the SMU may be obtained.

In 508, the CCC may be determined from the value of the current, thevalue of the voltage, and a known characteristic of the capacitor.

In 510, the VCC may be determined from the value of the current, thevalue of the voltage, and the known characteristic of the capacitor.

In certain embodiments, the known characteristic of the capacitor may beone of a capacitance, an impedance, or a reactance. The term knowncharacteristic refers to a quantifiable characteristic that may beconsidered stable for the useful life of the capacitor. Thus, in oneembodiment, the capacitor may be characterized over one or moreoperating frequencies. In other words, the known characteristic maychange in proportion to operating frequency of an excitation signal andthe known characteristic may be determined for one or more specifiedoperating frequencies.

FIGS. 6A-6B—Exemplary Current and Voltage Response Signal Waveforms

FIGS. 6A and 6B illustrate exemplary voltage and current signalsresponsive to an excitation signal applied to a capacitor by an SMUaccording to one or more embodiments of the present invention. Theexcitation signals may be generated by an SMU as described above, suchas SMU 200. The SMU may perform one or more of the methods describedabove, in whole or in part. Further the SMU may perform variouscombinations of various elements of the methods described above.

As illustrated in FIG. 6A, an SMU may be operated with a firstcurrent-range setting and a first voltage-range setting. As shown, theSMU may operate with a 6V voltage-range setting and a 1 mA current-rangesetting. Additionally, a trapezoidal excitation signal with a 5Vmagnitude may be applied with a corresponding current signal limited to1 mA. In other words, the excitation signal may be applied to acapacitor by an SMU with the SMU operating with a first current-rangesetting and a first voltage-range setting. Responsive to the appliedexcitation signal, voltage and current waveforms as illustrated in FIG.6A may be generated.

As illustrated in FIG. 6B, the SMU may be operated with a secondcurrent-range setting and the first voltage-range setting. As shown, theSMU may operate with a 6V voltage-range setting and a 10 mAcurrent-range setting. Additionally, a trapezoidal excitation signalwith a 5V magnitude may be applied with a corresponding current signallimited to 1 mA. In other words, the excitation signal may be applied toa capacitor by an SMU with the SMU operating with a second current-rangesetting and a first voltage-range setting. Responsive to the appliedexcitation signal, voltage and current waveforms as illustrated in FIG.6B may be generated.

As can be seen from FIGS. 6A and 6B the waveforms are not identical.Thus, the magnitudes of the current waveforms cannot be directlycompared to determine the error in the second current-range setting,e.g., the 10 mA range. In order to compare the current waveforms, themagnitudes of the voltages may be scaled.

One method to scale the voltages may be to compare the impedancescomputed from the magnitudes of the current and voltage. In other words,compare a value of the current to the value of the voltage. In oneembodiment, the magnitudes may be determined from the standarddeviations of the voltage and current measurements. Assuming the firstcurrent-range setting, e.g., the 1 mA range is previously calibrated,then the degree of mis-calibration of the second current-range setting,e.g., the 10 mA range, may be determined from the ratio of theimpedances. Note that, in embodiments where the determined value of thevoltages are equal, the degree of mis-calibration may be determined bycomparing the determined values of the current.

Additionally, in embodiments in which the excitation signal has atrapezoidal waveform, the excitation signal may have components atmultiple frequencies. In such embodiments, the resulting impedance maybe a weighted impedance over multiple frequencies. Note further, that incertain embodiments, the weighting may change from one current-rangesetting to another due to frequency response changes. Thus, in certainembodiments, the impedance of the capacitor at fundamental frequency ofthe excitation signal may be determined by performing an FFT on both thecurrent and voltage waveforms. Then, the complex values representing thefundamental frequency of each voltage and current may be used todetermine impedances. Additionally, the reactance may be determined. Incertain embodiments, the reactance may be used to determine themis-calibration.

In certain embodiments, it may be necessary to correct for inductancewhich may be in series with the capacitor. The inductance may be real,such as interconnect inductance, or virtual, such as an artifact of thefrequency response of the signal path. Similar to capacitance,inductance may contribute to the imaginary term of the impedance.Further, if the inductance changes with current-range settings of theSMU, then the inductance may introduce error in the computationsdescribed above. However, since an inductance change may bedistinguished from a capacitance change over a range of frequencies, thecapacitance may be calculated at other frequencies of the excitationsignal, such as, for example, the third and fifth harmonic of theexcitation signal. Note, it is envisioned that other harmonics may beused.

It should further be noted that the various terms or designations forcircuits/components and signals as they appear herein, for example insuch expressions as “driver circuit”, “delay circuit”, “data signal”,“control signal”, “first current”, “second voltage”, “firstcharacteristic”, etc. are merely names or identifiers used todistinguish among the different circuits/components and/or betweendifferent signals, currents, voltages, etc., and these identifying termsare not intended to connote any specific meaning, unless explicitlynoted otherwise.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

We claim:
 1. A method for calibrating and operating a source-measureunit (SMU), the method comprising: applying an excitation signal by theSMU to a capacitor; and obtaining one or more of: a current calibrationcoefficient (CCC) corresponding to a current-range setting of the SMU,said obtaining the CCC comprising: determining a value of a firstcurrent developed in a capacitor responsive to the excitation signal anda value of a second current developed in the capacitor responsive to theexcitation signal; and determining the CCC from the value of the firstcurrent and the value of the second current; or a voltage calibrationcoefficient (VCC) corresponding to a voltage-range setting of the SMU,said obtaining the VCC comprising: determining a value of a firstvoltage developed across the capacitor responsive to the excitationsignal and a value of a second voltage developed across the capacitorresponsive to the excitation signal; and determining the VCC from thevalue of the first voltage and the value of the second voltage.
 2. Themethod of claim 1; wherein when obtaining the CCC: said obtaining theCCC further comprises determining a value of a third voltage developedacross the capacitor corresponding to the value of the first current anda value of a fourth voltage developed across the capacitor correspondingto the value of the second current; and said determining the CCC furthercomprises determining the CCC from the ratio of value of the firstcurrent to the value of third voltage and the ratio of the value of thesecond current to the value of the fourth voltage; and wherein whenobtaining the VCC: said obtaining the VCC further comprises determininga value of a third current developed in the capacitor corresponding tothe value of the first voltage and a value of a fourth current developedin the capacitor corresponding to the value of the fourth current; andsaid determining the VCC further comprises determining the VCC from theratio of value of the third current to the value of first voltage andthe ratio of the value of the fourth current to the value of the secondvoltage.
 3. The method of claim 1; wherein when obtaining the CCC, saiddetermining the CCC further comprises determining the CCC based on afirst characteristic of the capacitor corresponding to the value of thefirst current and a second characteristic of the capacitor correspondingto the value of the second current; and wherein when obtaining the VCC,said determining the VCC further comprises determining the VCC based ona third characteristic of the capacitor corresponding to the value ofthe first voltage and a fourth characteristic of the capacitorcorresponding to the value of the second voltage.
 4. The method of claim3, wherein the characteristic of the capacitor includes one of: animpedance; a reactance; or a capacitance.
 5. The method of claim 1;wherein the excitation may have an amplitude and an operating frequency;wherein when obtaining the CCC said determining the CCC furthercomprises determining the CCC based on a known characteristic of thecapacitor and the value of the first and second currents, wherein theknown characteristic of the capacitor is based on the operatingfrequency of the excitation signal; and wherein when obtaining the VCCsaid determining the VCC further comprises determining the VCC based onthe known characteristic of the capacitor and the value of the first andsecond voltages.
 6. The method of claim 5; wherein the knowncharacteristic is one of: an impedance; a reactance; or a capacitance;and wherein the method further comprises determining the knowncharacteristic of the capacitor for a specified excitation signal,wherein the specified excitation signal has an operating frequency. 7.The method of claim 1; wherein said determining the value of the firstcurrent and said determining the value of the first voltage eachcomprises operating the SMU with a first current-range setting and afirst voltage-range setting; wherein said determining the value of thesecond current comprises operating the SMU with a second current-rangesetting and the first voltage-range setting; wherein the CCC correspondsto the second current-range setting; and wherein said determining thevalue of the second voltage comprises operating the SMU with the firstcurrent-range setting and a second voltage-range setting; and whereinthe VCC corresponds to the second voltage-range setting.
 8. The methodof claim 1; wherein the excitation signal comprises a trapezoidalwaveform.
 9. The method of claim 1; wherein the amplitude and operatingfrequency of the excitation signal are specified to maximize asignal-to-noise ratio.
 10. The method of claim 1; wherein the excitationsignal is a first excitation signal having a first operating frequency,wherein the CCC is a first CCC, wherein the VCC is a first VCC, andwherein the method further comprises: obtaining one or more of: a secondCCC corresponding to a third current-range setting of the SMU, saidobtaining the second CCC comprising: applying a second excitation signalto the capacitor comprising a second operating frequency, wherein athird current and a fourth current are developed in the capacitorresponsive to the second excitation signal; determining a value of thethird current developed in the capacitor and a value of the fourthcurrent developed in the capacitor; and determining the second CCC fromthe value of the third current and the value of the fourth current; or asecond VCC corresponding to a third voltage-range setting of the SMU,said obtaining the second VCC comprising: applying the second excitationsignal to the capacitor, wherein a third voltage and a fourth voltageare developed across the capacitor responsive to the second excitationsignal; determining a value of the third voltage developed across thecapacitor and a value of the fourth voltage developed across thecapacitor; and determining the second VCC from the value of the thirdvoltage and the value of the fourth voltage.
 11. The method of claim 10;wherein said determining the value of the third current and saiddetermining the value of the third voltage each comprises operating theSMU with the first current-range setting and the first voltage-rangesetting; wherein said determining the value of the fourth currentcomprises operating the SMU with the third current-range setting and thefirst voltage-range setting; and wherein said determining the value ofthe fourth voltage comprises operating the SMU with the firstcurrent-range setting and the third voltage-range setting.
 12. Themethod of claim 10; wherein the second excitation signal has a secondamplitude.
 13. The method of claim 1; wherein the capacitor has one ormore impedances corresponding to one or more frequencies of theexcitation signal, the method further comprising: weighting each of theone or more impedances to offset a corresponding one or more apparentseries losses in the circuit, wherein the one or more apparent serieslosses are a function of the one or more frequencies of the excitationsignal.
 14. A method for calibrating and operating a source-measure unit(SMU), the method comprising: applying an excitation signal by the SMUto a capacitor; determining a value of a current developed in thecapacitor responsive to the excitation signal; determining a value of avoltage developed across the capacitor responsive to the excitationsignal; and obtaining one or more of: a current calibration coefficient(CCC) corresponding to a current-range setting of the SMU, saidobtaining the CCC comprising determining the CCC from the value of thecurrent, the value of the voltage, and a known characteristic of thecapacitor; or a voltage calibration coefficient (VCC) corresponding to avoltage-range setting of the SMU, said obtaining the VCC comprisingdetermining the VCC from the value of the voltage, the value of thecurrent, and the known characteristic of the capacitor.
 15. The methodof claim 14, wherein the known characteristic of the capacitor is oneof: an impedance; a reactance; or a capacitance.
 16. The method of claim14; wherein the excitation signal comprises a trapezoidal waveform. 17.The method of claim 14; wherein the amplitude and operating frequency ofthe excitation signal are specified to maximize a signal-to-noise ratio.18. The method of claim 14, wherein the capacitor has one or more knowncharacteristics corresponding to one or more frequencies of theexcitation signal, the method further comprising: weighting each of theone or more known characteristics to offset a corresponding one or moreapparent series losses in the circuit, wherein the one or more apparentseries losses are a function of the one or more frequencies of theexcitation signal.
 19. The method of claim 14, further comprisingdetermining the known characteristic of the capacitor for a specifiedexcitation signal, wherein the specified excitation signal has anoperating frequency.
 20. A source-measure unit (SMU) comprising: afunctional unit; and output terminals, selectively coupled to thefunctional unit; and wherein the functional unit is configured to:obtain one or more of: a current calibration coefficient (CCC)corresponding to a current-range setting of a source-measure unit (SMU),wherein to obtain the CCC the functional unit is configured to:determine a value of a first current developed in a capacitor responsiveto an excitation signal applied by the SMU to the capacitor and a valueof a second current developed in the capacitor responsive to anexcitation signal; and determine the CCC from the value of the firstcurrent and the value of the second current; or a voltage calibrationcoefficient (VCC) corresponding to a voltage-range setting of the SMU,wherein to obtain the VCC the functional unit is configured to:determine a value of a first voltage developed across the capacitorresponsive to an excitation signal and a value of a second voltagedeveloped across the capacitor responsive to an excitation signal; anddetermine the VCC from the value of the first voltage and the value ofthe second voltage.